We report the demonstration of a sensitive absolute gravity gradiometer based on light-pulse atom interference techniques. The gradiometer consists of two absolute accelerometers operated in a differential mode. We report a differential acceleration sensitivity of 4 × 10 −9 g/Hz 1/2 and an inferred differential acceleration accuracy of less than 10 −9 g. This corresponds to a gravity gradient sensitivity of 4 E/Hz 1/2 (1 E = 10 −9 s −2 ) and an accuracy of better than 1 E for a 10 m separation between accelerometers. We demonstrate that the instrument can be used to detect nearby masses in a vibrationally noisy environment and characterize instrument sensitivity to spurious acceleration and rotation noise.

We measured the Newtonian constant of gravity, G , using a gravity gradiometer based on atom interferometry. The gradiometer measures the differential acceleration of two samples of laser-cooled Cs atoms. The change in gravitational field along one dimension is measured when a well-characterized Pb mass is displaced. Here, we report a value of G = 6.693 × 10 –11 cubic meters per kilogram second squared, with a standard error of the mean of ±0.027 × 10 –11 and a systematic error of ±0.021 × 10 –11 cubic meters per kilogram second squared. The possibility that unknown systematic errors still exist in traditional measurements makes it important to measure G with independent methods.

We report measurements in cavity QED of a wave-particle correlation function which records the conditional time evolution of the field of a fraction of a photon. Detection of a photon prepares a state of well-defined phase that evolves back to equilibrium via a damped vacuum Rabi oscillation. We record the regression of the field amplitude. The recorded correlation function is nonclassical and provides an efficiency independent path to the spectrum of squeezing. Nonclassicality is observed even when the intensity fluctuations are classical.